If your thermostat is a Blueface thermostat, the installation's master thermostat will display the mode icon (top left-hand corner) in white, while the installation's other thermostats will display it in grey.

If it is a Think thermostat, the installation's master thermostat allows the mode to be changed by pressing on the settings button (image of button). If the menu appears which allows you to change the installation mode after pressing this button, then that is the master thermostat.